CODEWITHSUNDEEP
kubernetespersistent-storagepersistent-volumeskubernetes-statefulset
S.Au.Ra.B.H
S.Au.Ra.B.H

Reputation: 457

Kubernetes: What is the significance of "storage" field in a block Persistent Volume?

Scenario

I have a PersistentVolume with volumeModeas Block. It is defined as:

apiVersion: v1
kind: PersistentVolume
metadata:
  name: block-vol
spec:
  accessModes:
  - ReadWriteOnce
  capacity:
    storage: 1Gi
  local:
    path: /dev/sdb # this path on the host specified below is used as a device mount
  nodeAffinity:
    required:
      nodeSelectorTerms:
      - matchExpressions:
        - key: kubernetes.io/hostname
          operator: In
          values:
          - <my-host>
  persistentVolumeReclaimPolicy: Retain
  storageClassName: block-storage
  volumeMode: Block

When I mount this on a statefulset with a VolumeClaimTemplate, I specify it's storage field as 1Gi. However, when exec'd in to the deployed pod, I see that the block size more than 1Gi (It is the actual size of that device on physical machine)

StatefulSet YAML:


apiVersion: apps/v1
kind: StatefulSet
metadata:
  name: nginx
spec:
  selector:
    matchLabels:
      app: nginx
  serviceName: "nginx"
  replicas: 1
  template:
    metadata:
      labels:
        app: nginx
    spec:
      terminationGracePeriodSeconds: 10
      containers:
      - name: nginx
        image: k8s.gcr.io/nginx-slim:0.8
        ports:
        - containerPort: 80
          name: web
        volumeDevices:
        - name: rawdev0
          devicePath: /dev/kdb0
  volumeClaimTemplates:
  - metadata:
      name: rawdev0
    spec:
      accessModes: [ "ReadWriteOnce" ]
      storageClassName: block-storage
      volumeMode: Block
      resources:
        requests:
          storage: 1Gi

I have used blockdev to find the size of block in bytes:

root@nginx-0:/# ls -lhrt /dev/kdb0 
brw-rw----. 1 root disk 8, 16 Jan 13 19:49 /dev/kdb0

root@nginx-0:/# blockdev --getsize64 /dev/kdb0 
536870912000 #size of block in bytes

Question

What does the storage field signify in this case?

Upvotes: 0

Views: 444

Answers (1)

Jonas
Jonas

Reputation: 128955

Kubernetes can't do much about the storage size for local volumes. The admin that created the PersistentVolume must set a proper size, for granular sizing he/she should probably create its own partition instead of mapping the local volume to a directory.

The storage size in the PersistentVolumeClaim is a request so that the app at least get a volume of that size.

Upvotes: 2

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